1. Technical Field
The present invention relates to a liquid ejecting apparatus, such as an ink jet type recording apparatus, particularly a liquid ejecting apparatus that ejects liquid in a pressure chamber from nozzles by driving a pressure generating unit.
2. Related Art
A liquid ejecting apparatus is an apparatus equipped with a liquid ejecting head and ejecting various kinds of liquid from the ejecting head. For example, there are image recording apparatuses, such as an ink jet type printer or an ink jet type plotter, as the liquid ejecting apparatus, and recently, liquid ejecting apparatuses are used for various manufacturing apparatuses, using the feature that it is possible to accurately land a very small amount of ink to a predetermined position. For example, the liquid ejecting apparatus is used for a display manufacturing apparatus that manufactures a color filter, such as a liquid crystal display, an electrode forming apparatus that forms electrodes of an organic EL (Electro Luminescence) display or an FED (Field Emission Display), and a chip manufacturing apparatus that manufactures a biochip (biochemical element). Further, the recording head for the image recording apparatus ejects liquid-state ink and color material ejecting heads for the display manufacturing apparatus eject liquid of R (Red), G (Green), and B (Blue) color materials, respectively. Further, the electrode material ejecting head for the electrode forming apparatus ejects an electrode material and the bioorganic material ejecting head for the chip manufacturing apparatus ejects a solution of a bioorganic material.
Recently, there is a tendency for the recording head used for the printer or the like to reduce the amount of ink ejected from the nozzles due to the demand for improvement in image quality. The initial speed of droplets is set to be high to reliably land a very small amount of droplets onto a recording medium. Accordingly, the droplets ejected from the nozzle extend during scattering and are separated into main droplets at the front and satellite droplets (sub-droplets) behind the main droplets. Some or all of the satellite droplets rapidly decrease in speed due to viscous resistance of the air and change into mist, failing to reach the recording medium. Accordingly, the satellite droplet changed into mist (ink mist) contaminates the inside of the apparatus and adheres to members that are easily charged, such as the recording head or the electric circuit, thereby causing errors in operation.
It has been attempted to actively attract droplets to a support member (or a platen or a base member) supporting a recording medium during recording and land the droplets onto the recording medium by generating an electric field between a nozzle formation surface of a recording head and the support member while charging the droplets ejected from nozzles, in order to prevent the inconvenience (for example, see JP-A-10-278252 or JP-A-2004-202867).
However, as shown in the schematic view of FIG. 9A, while the ink ejected from a nozzle 64 of a recording head grows toward a recording medium P and a support member 65, negative charges are induced at the front portion (the portion that becomes a main droplet Md) close to the support member 65 by electrostatic induction from the support member 65 that has been positively charged, whereas positive charges are induced at the rear end portion close to the opposite nozzle 64. Further, as shown in FIG. 9B, when ink ejected from a nozzle is, for example, separated into main droplets Md, a first satellite droplet Sd1, and a second satellite droplet (mist) Sd2, the main droplet Md is negatively charged, the second satellite droplet Sd2 is positively charged, and the first satellite droplet Sd1 is not charged. In this case, even if the main droplet Md and the first satellite droplet Sd1 are landed on the recording medium P, the second satellite droplet Sd2 is repelled from the positively-charged support member 65 and changes into mist around the nozzle formation surface of the recording medium. Some of the mist adheres to the nozzle formation surface. When mist adheres to the nozzle formation surface, it is necessary to regularly sweep the nozzle formation surface with a wiping member. Further, the mist that does not adhere to the nozzle formation surface may adhere to other components of the printer which have different polarity from the mist and contaminate them.
Accordingly, a configuration that keeps a positively-charged satellite droplet away from a nozzle formation surface (makes a positively-charged satellite droplet travel onto a recording medium) by disposing an electrode around a nozzle, changing the polarity of the nozzle when ink starts to be ejected from the nozzle, for example, from positive to negative, and changing again the polarity of the electrode from positive at the timing when the ink ejected from the nozzle is separated into main droplets and satellite droplets, has been proposed (for example, see JP-A-2010-214652). Further, a configuration that lands droplets onto a recording medium by ejecting ink from a nozzle with a support member (base member) negatively charged, changing the polarity of the support member into positive, allowing main droplets to be landed onto the recording medium by the inertial force, and attracting satellite droplets or mist to the support member, which is charged with the opposite polarity to that of the satellite droplets or the mist, at the timing when the ink is separated into the main droplets and the satellite droplets, has been proposed (for example, see JP-A-2010-214880).
However, recently, as the driving frequency for ejecting ink becomes higher in the type of printer, the next ink is ejected from the nozzle before the satellite droplets land on the recording medium in some cases. Therefore, in the configuration of changing the polarity of the electrode at the timing of ejecting ink or the timing of separating the ink, it is more difficult to reliably land the satellite droplets to the recording medium and scattering of the ink is influenced, thereby making the landing unstable.
Further, a configuration that prevents an electric field from being generated between the nozzle formation surface and the support member may be considered to prevent the ink from being charged, but it has been known that the ejected ink is charged even though the ink is ejected from a nozzle in the configuration. That is, for example, as in the schematic view shown in FIG. 10, in the configuration of ejecting ink to the recording medium P from a nozzle 71 by generating a pressure change of ink in a pressure chamber 70 and using the pressure change, by applying driving voltage to a driving electrode 69 of a piezoelectric vibrator 68 of a recording head, when piezoelectric voltage is input to the driving electrode 69 of the piezoelectric vibrator 68, the piezoelectric vibrator 68 and the pressure chamber 70 are insulated, such that negative charges are induced in the ink in the pressure chamber 70 around the piezoelectric vibrator 68 by electrostatic induction. Further, positive charges are induced in the ink around the nozzle 71, opposite to the piezoelectric vibrator 68. In a common recording head, a nozzle formation surface 72 is grounded, such that the positive charges of the ink move to the nozzle formation surface 72, but, as described above, in the configuration of ejecting ink at a higher driving frequency, ink is ejected from the nozzle 71 with positive charges slightly remaining. As a result, the ink ejected from the nozzle 71 is positively charged.
Further, the ink ejected from the nozzle 71 has a tendency to be more positively charged (negative decreases when negatively charged and then ejected) by Lenard effect while scattered toward the recording medium P. That is, when the ink is charged, the positive charges collect to the center portion of the droplet, while the negative charges collect to the surface portion. Further, the droplets are gradually made positive by vaporization or separation of the surface portion during scattering.
As described above, since the ink ejected from the nozzle is charged even in the configuration that does not generate an electric field between the nozzle formation surface and the support member, mist adheres to the nozzle formation surface or the components of the printer.
The phenomenon described above is not limited to the piezoelectric vibrator and is also generated in other pressure generating units that are operated by applying a driving voltage, such as a heater element.